In semiconductor wafer processing, inductively coupled plasmas (ICPS) have been used with success in plasma processing, particularly plasma etching, of semiconductor wafer substrates. One processing apparatus for etching semiconductor wafers with an ICP is disclosed in Bayer et al. U.S. Pat. No. 5,569,363, hereby expressly incorporated by reference herein. In the Bayer et al. patent, RF energy is applied to a coil that surrounds a vacuum chamber outside of a chamber wall made of quartz. The coil generates a magnetic field within the chamber which excites electrons and forms a plasma. While the field is largely inductively coupled, the development of a high peak to peak voltage across the coil causes some degree of capacitive coupling of energy into the chamber. Such voltage has, in situations, developed a sheath of 900 to 1000 volts, for example, which imparts substantial energy to ions in the plasma. Such energy is enough to cause an undesirable degree of sputtering of the inside of the chamber wall and the sheath reduces the useful volume of the plasma.
It has been proposed to add a metal Faraday shield around the outside of the chamber, between the coil and the chamber wall. The proposed shield is provided with slits that extend axially but which leave a complete circumferential band around each edge of the shield. The shield is grounded to prevent capacitive coupling of voltage into the chamber while the slits prevent the inductive shielding of magnetic energy that is required to energize the plasma. Such a proposed shield, unfortunately, has the effect of preventing initial ignition of the plasma by the coil, requiring a separate electrode or discharge element to start the plasma.
There is a need, therefore, to provide for the shielding of a chamber from capacitive coupling effects when a coil is employed to inductively couple energy into a chamber, and to do so without interfering with plasma ignition.